I've seen the Frog Sac but never actually used one...the NS goes with me on every trip and I frequently wear it. Here are my thoughts:
Frog Sac doesn't have arm holes (at least the older models didn't...maybe the new ones do), but the NS is like a poncho so your arms can stay out while your torso is covered. But the Frog Sac can be vented by not zipping it up all the way...the NS has only a head hole.
You can wear a jacket over the NS for total warmth and to protect the NS - no arm holes makes this difficult w/ the Frog Sac.
The Frog Sac covers the legs but the NS does not.
The NS is thicker...not better or worse, depending on the conditions you want to use it in.
NS is down and Frog Sac is synthetic - standard pros/cons apply.
Personally, I think the Frog Sac would be too cumbersome to wear around camp for anything more than a pee break or sitting around talking. Same with the PeaPod - I never wear that when I take it, either. I wear the NS under my rainjacket for all kinds of camp chores.
Hope this helps.
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